Your Guide to Pregnancy Hormones

Updated: August 24, 2016

Lucas Ottone/Stocksy

With the possible exception of puberty, you've never been so affected by these rampaging chemicals—so get to know (and respect) pregnancy hormones now.

It doesn't seem quite fair to accuse your hormones of causing every bloat, blemish and emotional outburst during pregnancy. But the truth is, the potent cocktail of chemicals really is guilty as charged. And everything that's happening to your body these days — both the good and the not-so-good — can be pinned on them. Here are the important hormones involved in your pregnancy.

Understanding Pregnancy Hormones

The main pregnancy hormones include:

FSH: Follicle stimulating hormone is the first in a cascade of hormones that’s necessary to launch your pregnancy and is present before you even conceive.

LH: Luteinizing hormone works in concert with FSH to orchestrate your menstrual cycle. Both FSH and LH are inactive during pregnancy itself.

hCG: Once egg meets sperm, human chorionic gonadotropin steps in to amp up the production of estrogen and progesterone. It also suppresses your immune system to support your growing baby.

Estrogen: Stimulated first by hCG and later by the placenta, estrogen helps the uterus grow, regulates the production of other key hormones, and triggers the development of baby’s organs, among other functions.

Progesterone: Similarly to estrogen, this pregnancy hormone is first triggered by hCG and later the placenta. Among its many other purposes, progesterone encourages breast tissue growth and later helps soften ligaments and cartilage to prepare you for labor.

Relaxin: This hormone is known mostly for relaxing your muscles, bones, ligaments and joints later in pregnancy in preparation for labor.

HPL: Human placental lactogen prepares your breasts to breastfeed. Along with placental growth factor, it’s produced by the placenta to adjust your body’s metabolism to feed your baby. In some women, HPL and placental growth factor are thought to block insulin, resulting in gestational diabetes.

Oxytocin: Though it’s present throughout your pregnancy, this muscle-contracting hormone is mostly known for stimulating labor contractions.

Prolactin: This hormone causes your breasts to increase in size and eventually produce milk.

So which hormone has the most influence on a safe pregnancy? While all pregnancy hormones are necessary, a few play an outsized role. hCG gets things rolling and is what your doctor first tests for to confirm your pregnancy. Once it does its job, estrogen and progesterone play an especially vital role during the first and second trimesters in triggering fetal development as well as common pregnancy symptoms.

Read on to learn more about the hormones responsible for launching your pregnancy.

Follicle stimulating hormone (FSH)

At the beginning of your cycle, just as the last of your old uterine lining is sloughed away in your period, follicle stimulating hormone (FSH) stimulates one of your ovaries' egg-bearing follicles to ripen and start making the hormone estrogen. Estrogen tells the uterine lining to begin rebuilding and, when you conceive, eventually shuts down further secretion of FSH (why you don’t ovulate while you’re pregnant).

Women who have fraternal twins often have higher levels of FSH, which makes them naturally more likely to drop more than one egg. That includes women over 35—which is often the reason why older moms are more likely to have twins.

Luteinizing hormone (LH)

As FSH starts the production of estrogen, estrogen sets off a blast of luteinizing hormone (LH), which bursts the follicle and releases an egg. The splayed-apart follicle now becomes a structure called the corpus luteum, which is normally programmed to disintegrate in about 14 days. When it does, diminishing hormone levels trigger your period.

But if a sperm and egg meet up, the corpus luteum hangs around instead of disintegrating, continuing to grow and produce enough hormones to nourish and support your baby. The progesterone pumped out by the corpus luteum fine-tunes the ripening of the uterus and inhibits LH until it gradually shrinks starting at about 6 weeks and the placenta takes over by about 12 weeks.

If you have trouble conceiving, your doctor may test your LH levels. When they’re higher than normal it can mean ovulation isn’t happening or may be related to an overall imbalance in sex hormones (as there is with polycystic ovarian syndrome, a.k.a. PCOS).

Human chorionic gonadotropin (hCG)

You may not have known it at the time, but you already came face-to-stick with the hormone hCG from the day that little plus sign showed up on your home pregnancy test. This just-for-pregnancy hormone is found in your urine and your blood, which explains why you can pee on a stick and why your practitioner may run a blood test to find out for sure.

hCG is created by the cells of the newly developing placenta within days after a fertilized egg implants in the uterine lining. It stimulates the corpus luteum to pump out more estrogen and progesterone. If you’re carrying more than one baby, expect even more of the hormone chugging through your bloodstream. No two women’s hCG levels are the same, and they can vary within a very normal wide range. Your practitioner may check its levels to monitor your baby's progress.

Very early in pregnancy, your hCG level is quite low. But within days it’ll soar, doubling every 48 hours or so (give or take). The rapid increase peaks somewhere between 7 and 12 weeks of pregnancy and then starts to decline at around the beginning of the second trimester. At this point the placenta takes over production of estrogen and progesterone—that’s thought to be why morning sickness and frequent urination tend to peak in the first trimester, when hCG levels are at their highest. But hCG is present throughout pregnancy. And since it also works to suppress your immune system to reduce the chance that your body will reject the baby, it’s also what makes you even more susceptible to colds and the flu.

Other pregnancy hormones

From the What to Expect editorial team and Heidi Murkoff, author of What to Expect When You're Expecting. Health information on this site is based on peer-reviewed medical journals and highly respected health organizations and institutions including ACOG (American College of Obstetricians and Gynecologists), CDC (Centers for Disease Control and Prevention) and AAP (American Academy of Pediatrics), as well as the What to Expect books by Heidi Murkoff.